Mendelian randomization analysis of psoriasis and psoriatic arthritis associated with risks of ulcerative colitis

Abstract Objective This study is designed to explore the potential causal relationship between psoriasis and psoriatic arthritis (PsA) while investigating the genetic basis shared by these inflammatory diseases. Methods Significant single nucleotide polymorphisms (SNPs) associated with UC, psoriasis, and PsA were selected as genetic instrumental variables using Genome‐Wide Association Study (GWAS) datasets. Additionally, Mendelian randomization (MR) methods, including inverse‐variance weighting (IVW), MR‐Egger regression, and Weighted Median (WME), were utilized to evaluate the causal relationships between these diseases. Moreover, sensitivity analysis and heterogeneity testing were conducted to validate the stability of the results. Results A total of 123 significant SNPs associated with psoriasis, PsA, and UC were identified as genetic instrumental variables based on GWAS datasets. The analysis revealed a 36% increased risk of UC with psoriasis (odds ratio [OR] = 1.350, 95% confidence interval [CI] = 1.065‐1.729, P = 0.012) and a 32.9% increased risk of UC with PsA (OR = 1.329, 95% CI = 1.176‐1.592, P < 0.001). Further analysis showed a 43.5% increased risk of psoriasis with UC (OR = 1.435, 95% CI = 1.274‐1.831, P < 0.001) and a 45.8% increased risk of PsA with UC (OR = 1.458, 95% CI = 1.166‐1.822, P = 0.0013). In addition, sensitivity analysis and heterogeneity testing demonstrated the high stability of these results. Particularly, neither MR‐Egger regression analysis nor leave‐one‐out analysis revealed significant heterogeneity or pleiotropy bias, indicating the reliability of these causal estimates. Moreover, the use of the MR‐PRESSO further confirmed the positive correlation between psoriasis and UC, and the corrected estimates remained consistent with IVW analysis results after excluding potential outlier SNPs, enhancing the credibility of the analysis. Conclusions This study strengthens the understanding of the genetic and causal relationships among UC, psoriasis, and PsA through GWAS and MR methods, revealing the genetic basis they may share. These findings not only provide a novel perspective on the comorbidity mechanisms of these diseases but also offer a valuable reference for the development of future treatment strategies and intervention measures.


INTRODUCTION
With unclear pathogenesis and the site of disease mainly involving the rectum, left colon, and even the entire colon, ulcerative colitis (UC) is clinically characterized by symptoms such as diarrhea, abdominal pain, and bloody mucus in the stool, with severe cases leading to complications including intestinal perforation and cancer.2][3] Psoriasis is an immune-mediated, polygenic hereditary skin disease that can occur in people of any age group, which often recurs and affects approximately 2%-3% of the global population.In recent years, various researchers have studied the correlation between psoriasis and UC, with some differences found in the results obtained. 4,5Some studies have found an increased incidence of UC in patients with psoriasis and psoriatic arthritis (PsA), while others suggest no significant correlation between psoriasis, PsA, and UC. 6 In addition, psoriasis and PsA are chronic inflammatory diseases that primarily affect the skin and joints.Specifically, psoriasis is characterized by red patches with silvery-white scales on the skin, while PsA manifests as joint inflammation and pain, significantly impacting the quality of life of patients. 7By contrast, UC is a chronic inflammatory bowel disease that affects the large intestine, with symptoms such as abdominal pain, diarrhea, and weight loss. 8Despite their differences in manifestation and affected organs, all three diseases are associated with abnormal activity of the immune system.
In recent years, with the rapid development of genetics and molecular biology research, scientists have begun to explore the genetic basis of these diseases and their potential connections.With the widespread application of Genome-Wide Association Studies (GWAS), methods such as Mendelian Randomization (MR) and Polygenic Risk Score (PRS) are increasingly adopted to discover disease etiology and causal inferences. 9Compared to traditional observational epidemiological studies, MR studies and PRS analyses are not influenced by common confounding factors and have a reasonable causal timeline.
Specifically, MR is an analytical method that uses genetic instrumental variables (such as SNPs) to assess potential associations between exposure and outcomes, 10 which evaluates causal relationships based on 3 core assumptions: (1) The selected genetic variants must be related to the exposure of interest; (2) These genetic variants should not be associated with any potential confounders; (3) These variants can only influence the outcome through exposure instead of other pathways.
These three assumptions ensure the validity of causal interpretations in MR analysis.Meanwhile, compared to observational studies, MR has fewer confounding factors and can reverse causal biases. 11Additionally, a meta-analysis revealed a particularly significant association between psoriasis and UC in children and adolescents, with significant associations in both directions.However, conclusions regarding causality cannot be directly drawn based solely on associations present in observational study designs, which are often retrospective or crosssectional studies with limited sample sizes and confounding factors.
For example, UC patients receiving anti-TNF-α treatment may be susceptible to paradoxical psoriasis side effects, making the medication a risk factor for psoriasis.Therefore, it is challenging to determine the causal relationship between PsA and UC, as this may exaggerate the connection between psoriasis and UC.
In this study, single nucleotide polymorphism (SNP) sites published in large-scale GWAS databases were utilized as genetic instrumental variables to explore the potential causal relationships between psoriasis, PsA, and UC using the MR method.

Selection of outcome GWAS datasets
Publicly available published studies or summaries of GWAS data were utilized, and no ethical approval was required due to no original data used in this study.However, these studies were approved by their corresponding academic ethics review committee, and each participant signed a written informed consent form.Genetic variants and data sources were selected.
Generally, an F > 10 indicates that the selected SNPs strongly predict modifiable risk factors for disease onset.

Instrumental variables
After obtaining the data from the website using R software and to avoid analysis bias due to strong linkage disequilibrium (LD) between SNPs, the screening criteria were as follows: (1) P < 5×10-5; (2) a physical distance of M > 100,00 kb between every two genes; (3) a threshold of r 2 < 0.001 for LD between genes, with an F value > 10.Finally, SNPs that were mutually independent and significantly associated with UC were selected as the final instrumental variables.

MR analysis
The analysis methods used in this study mainly included inversevariance weighted (IVW), MR-Egger regression, and Weighted Median (WME) from the Two-sample MR package.Specifically, IVW is the most commonly used method for calculating the weighted average of the effect values of all instrumental variables, and MR analysis relies on the IVW method for reliable results.By contrast, MR-Egger can perform MR analysis and apply it to assess pleiotropy, which utilizes the inverse of the outcome variance as weights for fitting.In the meantime, WME boasts the advantage of providing effective results in MR analysis even when at least half of the effective instrumental variables are available.

Sensitivity analysis
Heterogeneity testing examines differences between each IV, with P > 0.05 indicating no heterogeneity.Heterogeneity was evaluated using Cochran's Q test, with P < 0.05 indicating heterogeneity and P > 0.05 indicating no heterogeneity.Meanwhile, the reliability of MR analysis results was verified using the pleiotropy test, in which the intercept value of the MR-Egger method was applied, with the intercept value of P > 0.05 indicating no horizontal pleiotropy, and the MR analysis results were deemed unreliable if pleiotropy existed.Moreover, sensitivity was tested using the "Leave-one-out" method, where individual SNPs were successively removed to determine whether it was an outlier, thereby observing the stability of the outcome after removing each SNP.

Statistical analysis
Various MR analysis methods were employed, including IVW, Weighted Median (WM), MR-Egger, Weighted Model, and simple model.Additionally, the MR-PRESSO method was used to address issues of heterogeneity and pleiotropic effects.Results were presented as odds ratios (OR) and 95% confidence intervals (CI).Furthermore, the intercept value from MR-Egger regression was used to test for pleiotropy, and a phenotypic scanner was utilized to detect associations between genes and other diseases, so as to preclude the impact of genetic pleiotropy.

Instrumental variables
In this study, LD-independent SNPs for exposures (psoriasis, PsA, UC) were selected using R software based on screening criteria to select SNPs with genome-wide significance.A total of 123 SNPs were obtained as instrumental variables, with the top 15 significant SNPs listed in Table 1.The listed SNPs will be removed under the following conditions:

Causal effects of psoriasis and PsA on UC
Figure 1 shows the estimated causal effects of psoriasis and PsA on UC.
The results indicate that psoriasis is associated with a 36% increased risk of UC (IVW: OR = 1.350 (1.065-1.729),P = 0.012) (Table 2).Specifically, this association is consistent across methods of the Maximum Likelihood, Penalized Weighted Median, WM, and Weighted Model.
Since the MR assessment of PsA on UC indicated no heterogeneity (Table 1), the IVW under fixed effects was determined as the primary MR analysis method.Meanwhile, the primary MR analysis conducted using the IVW (fixed effects) method showed that PsA is associated with a 32.9% increased risk of UC (IVW (fixed effects): OR = 1.329 (1.176-1.592),P < 0.001) (Table 2).A scatter plot was used to demonstrate the estimated effect sizes of SNPs on exposure and outcomes (Figure 1).The "Leave-one-out" plot is shown in Figure 2.

Estimated causal effects of UC on psoriasis and PsA
It has been found that the genetic prediction of UC is positively associated with a 43.5% increased risk of psoriasis (IVW: OR 1.435

Sensitivity analysis
In this study, the instrumental variables were selected by strictly following the selection criteria, and individuals from the same ethnic population were included, resulting in a minimal likelihood of false negative results.Heterogeneity testing was conducted on the results, and the Q values and Q P-values for IVW and MR-Egger were 136.85 (0.132) and 138.76 (0.165), respectively, both of which exceeded 0.05, indicating no heterogeneity.The findings are visualized in Figure 5.
Moreover, various sensitivity analyses were conducted to further explore the potential pleiotropy between exposure and outcomes.
Specifically, the heterogeneity of instrumental variables was quantified using Cochran's Q test, where Ph < 0.05 indicates significant heterogeneity, while sensitivity analyses were performed without considering possible confounding SNPs.In the MR analysis, the heterogeneity testing showed heterogeneity was present except for the evaluation of PsA on UC (Table 1).Additionally, the MR-Egger intercept for all MR analyses showed no evidence of horizontal pleiotropy (Table 1).
Notably, the original estimate from MR-PRESSO indicated no association between psoriasis and UC.However, after excluding two SNPs, the corrected estimate yielded the opposite conclusion, consistent with the results from methods of IVW, Maximum Likelihood, Penalized Weighted Median, WM, and Weighted Model.The results from IVW or IVW (fixed effects) methods were consistent with the original and corrected estimates in MR-PRESSO, demonstrating the stability of the results (Table 3).

DISCUSSION
With the rapid progress in genetics and molecular biology in recent years, researchers have conducted in-depth exploration into the genetic associations among these diseases. 12Particularly, GWAS have provided powerful tools for identifying genetic variations associated with these diseases. 13Meanwhile, the Mendelian Randomization (MR) method further allows researchers to explore potential causal relationships between diseases by using these genetic variations as instrumental variables, thus overcoming confounding factors and reverse causation issues in traditional observational studies.In this study, the potential causal relationships between UC, psoriasis, and PsA were investigated using GWAS and MR.These diseases all belong to immune-mediated chronic inflammatory conditions, impacting the health and quality of life of millions of people globally, while early epidemiological studies and clinical observations have hinted at a possible association between them despite their differences in clinical manifestations and affected organs. 14,15This study found that psoriasis and PsA significantly increased the risk of UC, and conversely, UC also significantly increased the risk of psoriasis and PsA.In this regard, these results not only support findings from early observational studies but also provide more compelling evidence for causal inferences. of psoriasis. 16,17Both psoriasis and UC exhibit excessive differentiation of Th17 cells, with overexpression of cytokines such as TNF-α, IL-17A, and IL-23 in the serum. 18In particular, IL-23 is a key factor that induces Th17 cell differentiation leading to the secretion of IL17A and thus affecting keratinocytes, IL17 hypersecretion and action are also observed in the serum of patients with UC, acting on and affecting intestinal epithelial cells. 19Despite no in-depth exploration into specific molecular mechanisms in this study, the findings align with known pathophysiological data, for example, such diseases are associated with abnormal activation of Th17 cells and the IL-17 pathway, which are critical factors in regulating immune responses and inflam-matory reactions. 20Additionally, TNF-α, a pro-inflammatory cytokine, also significantly contributes to the development of these diseases. 21e genetic variations discovered in this study may play a crucial role in these diseases by regulating the expression or function of these pathways and factors.The existing research suggests that the correlation between psoriasis and UC also involves abnormalities in co-pathogenic genetic loci and disturbances in intestinal microbiota.
Meanwhile, the susceptibility gene loci for both psoriasis and UC have been found to be at the 6p2.1 locus, which includes the UC-associated Furthermore, certain specific bacterial strains, such as segmented filamentous bacteria, can induce the differentiation of Th17 cells through intestinal epithelial cells, ultimately disrupting the immune system. 22wever, this study also comes with some limitations despite providing valuable insights: (1) Although the application of the MR method reduced the impact of confounding factors, it assumed that the selected genetic variations only affected the outcomes through specific exposure pathways, a hypothesis that may be reversed under

( 1 )
SNPs related to outcomes and confounding factors will be excluded; (2) Specific SNPs that do not exist in the outcome GWAS and for which no LD proxies can be retrieved from the outcome GWAS during the extraction of specific SNPs will be excluded; (3) The effects of ambiguous or palindromic SNPs with inconsistent allele frequencies cannot be reversed; (4) F-statistic > 10 for IVs, indicating little evidence of weak instrument bias.
Psoriasis is closely related to T-cell immunity, and elevated levels of cytokines such as IL-17A and TNF-α can be detected in skin lesions and serum, in which the T-cell-mediated immune response significantly contribute to the onset and progression of psoriasis, with IL-23 and Th17 cells playing essential roles in the occurrence and maintenance F I G U R E 2 Leave-one-out analysis of the impact of risk of PsA on UC patients.
UC3 gene and psoriasis-associated PSORS1 gene.In addition, PsA and UC share a common susceptibility gene, SLC22A5 (5q31), which F I G U R E 3 Genetically predicted association between body and risk of Ps in UC patients.is involved in regulating intestinal absorption function.In comparison, other genes such as IL23R and IL-12B have been found to be involved in the pathogenesis of psoriasis and UC by affecting the IL23/Th17 axis and disrupting the immune system.14In recent years, research on the gut microbiota has revealed that psoriasis and UC can manifest as abnormalities in the composition of specific gut bacterial genera, including a significant increase in the ratio of Firmicutes to Bacteroidetes.Moreover, in-depth research on mechanisms has shown that gut dysbiosis can lead to impaired production of short-chain fatty acids, thus affecting intestinal permeability and promoting the secretion of inflammatory factors and the aggregation of neutrophils.

certain circumstances; ( 2 )
Since this study mainly relied on GWAS data from European populations, further validation for the generalizability of findings needs to be performed in other ethnic and geographic populations; (3) While observing the significant statistical correlations, experimental studies are still needed to further clarify specific molecular mechanisms and biological pathways.

5 CONCLUSIONF I G U R E 5
In conclusion, this study strengthens the understanding of the genetic and causal relationships among UC, psoriasis, and PsA through GWAS and MR methods, revealing the genetic basis they may share.These findings not only provide a novel perspective on the comorbidity mechanisms of these diseases but also offer a valuable reference for the development of future treatment strategies and intervention measures.Therefore, future research should further explore the specific molecular mechanisms among these diseases while considering F I G U R E 4 Leave-one-out analysis of the impact of body and risk of Ps in UC patients.Funnel plot of heterogeneity test results through MR analysis.
General data on SNPs associated with gastric ulcerative colitis.
TA B L E 1 Heterogeneity and pleiotropy analyses.
TA B L E 2a Heterogeneity test in Inverse-Variance Weighted method.bHeterogeneitytest in MR-Egger method.cPleiotropytest in MR-Egger intercept.UC: ulcerative colitis.(1.274-1.831),P<0.001), and such an association is consistent across the methods of the Maximum Likelihood, Penalized Weighted Median, WM, Simple Model, and Weighted Model.Additionally, the results show that UC is associated with a 45.8% increased risk of PsA (IVW: OR 1.458 (1.166-1.822),P=0.0013), with such an association consistent across the methods of the Maximum Likelihood, Penalized Weighted Median, WM, Simple Model, and Weighted Model.Moreover, scatter plots and "Leave-one-out" plots are shown in Figures3 and 4.